1. Field of the Invention
This invention relates to real image type variable magnification finder optical systems and, more particularly, finder optical systems in separation from the photographic system or of the external type which have made it possible to observe an excellent finder image. Still more particularly, it relates to real image type variable magnification finder optical systems suited to, for example, still cameras or video cameras.
2. Description of the Related Art
With the camera having the photographic system and the finder optical system formed in separation from each other, it has been desired, on making photographs, that when the photographic system is a variable magnification system, the finder field magnification varies with variation of the magnification of an image on the film plane. Since such a variable magnification finder is to be built in the camera, it is also desirable that its size is small, and its form allows a predetermined ratio of change of the magnification to be easily obtained.
It has been well known to employ the secondary image forming type in designing a variable magnification finder optical system. Of the parts constituting the variable magnification finder optical system, the objective lens is constructed with a large number of lens groups and predetermined ones of the large number of lens groups of this objective lens are arranged to axially move to effect varying of the magnification. This is called the real image type variable magnification finder optical system and has been proposed in many forms.
Of these forms of the real image type variable magnification finder optical systems, the external form of finder optical system which is constructed in separation from the photographic system has many merits generally over the TTL type finder optical system like that in the single-lens reflex camera. For example, the finder system itself can be minimized in size. Further, the construction and arrangement of the lens elements of the finder optical system can be made with relative ease. In recent years, therefore, this form is widely used in still cameras or video cameras.
As one of the real image type variable magnification finder optical system, Japanese Laid-Open Patent Application No. Sho 61-156019, for example, proposes a real image type variable finder optical system where the objective lens is constructed with a large number of lens groups and when varying the magnification, all its air separations are made to vary.
Another real image type variable magnification finder optical system in which variation of the magnification is performed in such a manner that one of the lens groups constituting the objective lens is moved and, at the same time, one of the lens groups constituting an erector lens is also made to move by the same distance as that the objective lens moves, as, for example, proposed in Japanese Patent Publication No. Sho 47-1912.
Further, there is proposed a 2-focal length changeover form of the real image type variable magnification finder optical system in, for example. Japanese Laid-Open Utility Model Application No. Sho 59-186832 where while the primary image plane by the objective lens is made stationary, an erector lens is interchanged by another one at a position where the magnification of the erector lens takes a value of, for example, -.sqroot.z, or -1/.sqroot.z (where z is the ratio of change of the magnification).
However, the real image type variable magnification finder optical system proposed in Japanese Laid-Open Patent Application No. Sho 61-156019 mentioned before has a problem that, if, as the magnification is varied by the objective lens, all the lens groups constituting the objective lens are arranged with addition of a condition that the total movement of each of the lens group is somewhat limited, the physical size of the objective lens is entirely increased and the total length of the lens becomes very long.
Also, in the real image type variable magnification finder optical system proposed in Japanese Patent Publication No. Sho 47-1912 mentioned before, it is difficult to increase the ratio of change of the magnification. There is another problem that as the ratio of change of the magnification increases, it becomes difficult to keep a constant diopter of the finder during the variation of the magnification.
Also, the real image type variable magnification finder optical system proposed in Japanese Laid-Open Utility Model Application No. Sho 59-186832 mentioned before, because the erector lens is interchanged with another one at positions where the magnification of the elector lens has values of -.sqroot.z and -1/.sqroot.z, a continuous variation of the magnification cannot be obtained, thus giving rise to a problem that its technique cannot be used except only in the 2-focal length changeover form.